1. Field of the Invention
This invention relates to a computer system.
2. Description of the Prior Art
Diagrammatically shown in FIG. 1 is a prior art computer system which comprises a central processor unit (CPU) 10, a storage control unit 20, a main storage 30, an input/output processor 40 and an external storage 50.
Since capacity of the main storage 30 is finite, the external storage 50 is required which can store data necessary for the CPU 10, and it is accessed for reading and writing. In the computer system shown in FIG. 1, the external storage 50 is accessed for "writing" or "reading" data from or to an area in the main storage 30 as follows.
More particularly, the CPU 10 commands the input/output processor 40 to execute an input/output initiation instruction under the control of the processor 40 by itself. An example of input/output processor 40 is shown in FIG. 2. A control 400 included in the input/output processor 40 receives the input/output initiation instruction and thereafter it is freed from intervention by the CPU to read from a given address in the main storage 30 via the storage control unit 20 an instruction indicative of the type of input/output operation such as "read" or "write", an address in the main storage 30 which is subject to input/output operation and the number of bytes to be transferred, thereby setting the read-out data into latches 401, 402 and 403. The input/output processor 40, freed from intervention by the CPU 10, uses the data in the three latches to access data in the external storage 50 in order to read or write the data from or into the main storage 30 via the storage control unit 20. When the CPU 10 completes the transmission of the input/output initiation instruction to the input/output processor 40, it is released from the controlling of the input/output operation and can participate in execution of another information processing in parallel with the input/output operation. When the input/output operation is terminated, the input/output processor 40 performs an input/output interruption to hand over the controlling to the CPU 10, thereby apprising the program of results of its operation.
According to the aforementioned system for data transfer between the external storage and the main storage, execution time of the program is obviously prolonged as compared to a system in which data necessary for computation are all stored in a main storage. Especially, in large-scale scientific technical calculations, it happens that all of the data in a matrix are not stored in a main storage. As a result, additional input/output operations are needed with respect to an external storage and these input/output operations require calculation time. Accordingly, the type of input/output operation has a great effect on the execution time of the program.
The prior art computer system has the following disadvantages in view of the type of input/output operation:
(a) Time is consumed before the CPU issues the input/output initiation instruction; and
(b) Time is consumed for the CPU to execute the interruption processing after the CPU receives the input/output interruption.
The above item (a) is due to the fact that the number of program steps is increased for searching and allotting a usable external storage connected to the input/output processor and searching a usable storage type such as disc or magnetic tape, and that positioning of the external storage is time consuming. Item (b) is due to the fact that the number of program steps is increased for processing the other competitive interruptions and the execution of the input/output interruption. Thus, time overhead is inevitably increased in the prior art system wherein input/output operations between the main storage and the external storage are effected through the input/output processor, and the execution of the input/output operations is initiated by the input/output initiation instruction and terminated by the input/output interruption.